The long-term goal of this research is to elucidate the molecular mechanisms underlying eukaryotic gene regulation. Focus is centered on the mechanisms by which human progesterone receptors (PR) cooperatively bind to complex promoters, and the role of hormone agonists and antagonists in regulating these reactions. A further goat is to determine the principles by which the associated structural transitions are propagated to neighboring domains. PR co-exist as two functionally distinct isoforms: an 83 kD A-receptor and a 99 kD B-receptor. The two isoforms are identical except that the B-receptor has an additional 164 amino acids at its N-terminus. The B-receptor often functions as a strong transcriptional activator while the A-receptor generally acts as a weak activator. It is hypothesized that this difference arises through the ability of the B-receptor to bind cooperatively at PR-regulated promoters. Mechanistically, the B-unique residues impose a hormone-dependent conformational constraint upon the remainder of the receptor. This constraint causes changes in PR structure and stability- changes that can include dramatic disorder-order transitions - resulting in cooperative DNA binding. It is proposed that a role of antagonists is to decouple these linkages by stabilizing ineffective conformations within the PR hormone-binding domain. This hypothesis and the underlying mechanism will be examined by carrying out the following studies: Aim 1 - The energetics of self-assembly for both isoforms in the presence and absence of progestin agonists and antagonists will be determined using analytical ultracentrifugation. Aim 2 - A rigorous thermodynamic analysis of the interactions of each PR isoform with the multi-site mouse mammary tumor virus promoter will be determined using quantitative DNAse footprinting. Aim 3 - The changes in isoform structure and stability associated with ligand and DNA-binding will be mapped using hydroxyl radical proteolytic footprinting, CD spectroscopy and microcalorimetry.